Wideband wireless systems operating at high frequencies, such as the 60 GHz frequency range, are able to realize high data rate transmissions in the order of Gigabits per second (Gbps). Wideband wireless systems are able to accomplish these high data rates through the use of very wide channel bandwidths because channel capacity (C) is proportional to channel bandwidth (B) as illustrated in the Shannon-Hartley channel-capacity theorem:C=B*Log2(1+S/N),where S/N is the signal-to-noise power ratio. Because input data sequences tend to be narrowband in nature, to take advantage of the high data-rate capabilities of wideband transmission, narrowband data signals are combined with a noise-like, pseudo-random number sequence that is known to both the transmitter and receiver to spread the data signal over a wide frequency band. The injection of such a “spreading sequence” enables high-speed transmission of the wideband data signals. The wideband data signals are decoupled from the known spreading sequence at the receiver, leaving the narrowband data signals for extraction.
FIG. 1 depicts a block diagram of a spread spectrum transmitter. Data signals 32 are transmitted by an antenna 34 via a transmission chain 36. As described above, in order to spread the data signals 32 over a wide frequency band, a spreading sequence provided by a spreading sequence generator 38 is combined with the data signals 32 at some point in the transmission chain 36 to produce a wideband signal to be transmitted via the antenna 34.
Different spread-spectrum techniques are distinguished according to the point in the transmission chain at which a spreading sequence is inserted in the communication channel. FIG. 2 depicts a block diagram of a system for injecting a spreading sequence at different points in a transmission chain. In FIG. 2, data 42 is transmitted by an antenna 44 via a modulator chain 46 and a power amplifier 48. If the spreading sequence is inserted at the data level, as shown at 50, the spectrum spreading is referred to as a direct sequence spread spectrum technique (DSSS). The modulation chain 46 receives the data 42 and a signal from a local oscillator 52. If the spreading sequence is incorporated at the carrier-frequency level, as shown at 54, the spectrum spreading is referred to as a frequency hopping spread spectrum technique (FHSS). Further, if the spreading sequence acts as an on/off gate to the transmitted signal at the power amplifier 48, as shown at 56, the spectrum spreading may be referred to as a time hopping spread spectrum technique (THSS).
A wideband signal may be transmitted as a single carrier signal or a multiple carrier signal, such as an orthogonal frequency-division multiplexing (OFDM) signal. Both single carrier and multiple carrier transmissions may implement the same basic packet format structure shown in FIG. 3. The packet 60 begins with a preamble portion 62 that provides training information to help receiver setup. The preamble portion may include data to assist the receiver: detect the current packet, adjust automatic gain control (AGC) settings, perform frequency and timing synchronizations, set a single carrier/multiple carrier parameter, set a header rate parameter, set a network ID number parameter, set a piconet ID number parameter, as well as setting other setup parameter. A header portion 64 provides information regarding parameters for decoding the packet payload portion 66 such that the receiver may adjust its decoding apparatus accordingly. The header portion 64 may include data regarding the length of the payload portion, modulation and coding methods, as well as other parameter data. The payload portion 66 contains the data sought to be transmitted from the transmitter to the receiver.
FIG. 4 depicts an example packet preamble format. A packet preamble 70 may include a packet synchronization sequence (SYNC) 72 that may be used for determining the start of the packet, frequency/timing synchronization, AGC setting, and other parameter transmission. A start frame delimiter (SFD) 74 may be included in the preamble as a timing reference for the remainder of the packet as well as transmission of other parameters. The channel estimation sequence (CES) 76 may be included for use in channel estimation at the receiver.
FIG. 5 depicts an example packet structure in the form of an 802.15.3c compliant single carrier frame specification. As noted above, the packet begins with a preamble portion 80 that includes a SYNC segment 82, an SFD segment 84, and a CES segment 86. A frame header portion 88 follows the preamble portion 80, and the frame header 88 is followed by a payload portion 90.
FIG. 6 depicts an example multiple carrier 802.15.3c OFDM frame format. It is noted that the time scale in FIG. 6 runs from right-to-left as indicated at 102. The OFDM packet begins with a preamble portion 104 that includes a SYNC segment 106, an SFD segment 108, and a CES segment 110. The preamble portion 104 is followed by a frame header portion 112 that precedes a data payload portion 114.
As described above, narrowband data signals are often spread over a wide bandwidth to take advantage of increased channel capacity available to wideband signals. FIG. 7 depicts an example spreading sequence and cover code plan for a preamble portion of a packet. The depicted preamble portion includes a SYNC segment 118 and an SFD segment 120. The SYNC segment 118 includes data signals combined with a spreading sequence 122 denoted as ‘a.’ Data is transmitted during the SYNC segment 118 in the form of a repeated cover code 124 that is combined with the spreading sequence, ‘a’ 122, to generate the wideband data signal. The repeated SYNC cover code 124 may include data instructing the receiver as to frequency/timing synchronization, AGC setting, as well as other parameters.
The SFD segment 120 may be transmitted using the same spreading sequence, ‘a,’ as is used for the SYNC segment 118 as noted at 126. The SFD segment 120 may include data conveyed via a cover code 128 that is combined with the spreading sequence 126 to generate the wideband data signal. The first segment of the SFD cover code 128 may be selected so as to generate a large phase shift between the last SYNC cover code segment and the first SFD cover code segment. This large phase shift may be detected by a receiver to identify a transition between the SYNC 118 and SFD 120 segments, and the large detected phase shift may be used as a timing reference for the remainder of the packet. Other data, including the length of the CES segment, may be transmitted via the SFD segment cover code 128.